Applications of radars for detecting human/object or the like are spreading in various fields. Among others, in order to aid safe driving of automobile, developments of devices for monitoring an obstacle or the like (target object) existing in the periphery of an automobile using a radar are in progress. As such an automobile peripheral monitoring radar, BSD (Blind Spot Detection) aiding blind spot detection, and CTA (Cross Traffic Alert) which generates an alarm when a person, an oncoming car, or the like exists at an intersection, and the like are being brought into practical use. Among these automobile peripheral monitoring radars, there are ones required to detect a target object in the range of a substantially fan shape constituted of a range of certain angle (for example, in a wide angle range of about −60° to +60° with the front of a radiation direction being a center). On the other hand, other than automobiles, there are cases where a wide-angle detection range is required similarly as an application example to an infrastructure intended for security purpose or monitoring purpose. In any case, increase in angle range is necessary, but simultaneously there may be cases where ones having no drop in characteristics within the angle range and ones which have symmetrical detection ranges are preferred.
Patent Document 1 discloses an array antenna with plural radiation patterns having main lobes in which radiation intensity peaks in plural directions and a sensor detecting a predetermined wide angle direction. For is this array antenna, there is presented a case example of power feeding in reverse phase as a feeding condition and about 0.5 and 0.2 as an amplitude ratio, where it is possible to form a radiation pattern in a wide angle direction instead of directivity toward the front.
Further, Patent Document 2 discloses a microstrip array antenna in which plural radiation elements are coupled by a directional coupler of ¼ wavelength side coupling form. As disclosed in the “Prior Art” section in this Patent Document 1, when a T-branched line of simple structure is used to constitute a power feeding circuit, due to the influence of radiation elements or reflection waves of power feeding lines, power distribution characteristics of the T-branched line deviates from a desired value, and an excitation distribution of respective radiation elements is disturbed from the desired value, which can deteriorate radiation characteristics of the antenna. However, the technology described in Patent Document 2 allows preventing such deterioration in radiation characteristics.